Conventionally, a nitride semiconductor light emitting element such as a light emitting diode (LED) or a semiconductor laser has a light emitting element structure in which a plurality of nitride semiconductor layers are epitaxially grown on a sapphire substrate in many cases. The nitride semiconductor layer is expressed by a general formula: Al1-x-yGaxInyN (0≦x≦1, 0≦y≦1, 0≦x+y≦1).
The light emitting element structure is a double heterostructure in which an active layer of the nitride semiconductor layer having a single-quantum-well (SQW) structure or a multi-quantum-well (MQW) structure is sandwiched between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. In a case where the active layer is an AlGaN based semiconductor layer, its AlN mole fraction (or Al composition ratio) is adjusted so that a bandgap energy can be adjusted to be in a range from a lower limit to an upper limit of bandgap energies of GaN and AlN (about 3.4 eV and about 6.3 eV), and the ultraviolet light emitting element having a light emission wavelength of about 200 nm to about 365 nm can be provided. Specifically, when a forward current flows from the p-type nitride semiconductor layer to the n-type nitride semiconductor layer, light is emitted in the active layer based on the above bandgap energy.
In order to externally supply the forward current, a p-electrode is provided on the p-type nitride semiconductor layer, and an n-electrode is provided on the n-type nitride semiconductor layer. FIG. 10 schematically shows an element structure of a general light emitting diode. The light emitting diode shown in FIG. 10 is configured such that an n-type nitride semiconductor layer 101, an active layer 102, and a p-type nitride semiconductor layer 103 are sequentially deposited on a template 100 formed by depositing a nitride semiconductor layer on a sapphire substrate, the p-type nitride semiconductor layer 103 and the active layer 102 are partially etched away until the n-type nitride semiconductor layer 101 is exposed, the n-electrode 104 is formed on the exposed surface of the n-type nitride semiconductor layer 101, and a p-electrode 105 is formed on a surface of the p-type nitride semiconductor layer 103.
In a case where the active layer is an AlGaN based semiconductor layer, each of the n-type nitride semiconductor layer and the p-type nitride semiconductor layer which sandwich the active layer is an AlGaN based semiconductor layer having an AlN mole fraction higher than that of the active layer. Therefore, in general, each of the n-electrode and the p-electrode is configured to have a laminated structure of two or more layers, in which its lower layer side is provided with a metal layer capable of forming ohmic contact with the n-type AlGaN based semiconductor layer or the p-type AlGaN based semiconductor layer, and its upper layer side is provided with an Au layer so that wire bonding can be performed with an Au wire or the like. As one example, the n-electrode has a four-layer structure of Ti/Al/Ti/Au formed from the lower layer side, and the p-electrode has a two-layer structure of Ni/Au formed from the lower layer side. Here, in a case where light emitted from the active layer is outputted from a rear-face side, when Al is contained in the n-electrode, light reflected from an interface on the rear-face side toward the active layer is reflected again by the n-electrode toward the rear-face side, so that external quantum efficiency of the light emitting element can be improved.
In a normal case, the n-electrode and the p-electrode are each subjected to a heat treatment such as annealing or sintering after deposited and patterned, in order to preferably form the ohmic contact between the metal layer on the lower layer side and the n-type or p-type AlGaN based semiconductor layer. However, due to the heat treatment, the metal layer on the lower layer side is alloyed with Au on the upper layer side, and the alloyed metal other than Au is exposed to a surface of each electrode and deteriorates a surface state, so that the wire bonding cannot be performed in some cases, and the deterioration becomes noticeable especially in a case where a heat treatment temperature is high.
Thus, a countermeasure against the deterioration due to the heat treatment on each electrode surface is conventionally taken such that the Au layer is formed again on each electrode surface by depositing and patterning a bonding layer of Ni and the Au layer. However, this countermeasure has the problem that production cost is increased because the number of the film forming steps is increased. Thus, Patent Document 1 shown below proposes that a barrier metal layer composed of metal having a melting point higher than that of Al is provided on the surface of the Al layer on the lower layer side to isolate the Au layer and the Al layer with the barrier metal layer, to prevent Au on the upper layer side from being alloyed with Al and Ti on the lower layer side.